1. Field of the Invention
This invention relates to lubricating oil compositions, particularly to lubricating oil compositions useful as functional fluids in systems requiring fluid coupling, hydraulic fluid, and/or lubrication of relatively moving parts. In a preferred embodiment, this invention relates to a lubricating oil composition useful as the functional fluid in automatic transmissions, particularly automatic transmissions used in passenger automobiles.
Automatic transmission fluids are required to have a variety of desirable characteristics besides acting as a satisfactory fluid coupling. Among these are allowing the transmission to shift smoothly, allowing the transmission to lock up during a shift from one speed to another within a certain specified period of time, and lubricating relatively moving parts such as bearing surfaces and clutch plates.
An automatic transmission is a complicated piece of machinery. It includes a turbine drive unit with a torque converter and one or more clutches which are engaged and disengaged automatically by an intricate hydraulic control unit. In a typical automatic transmission the clutches are made up of alternating steel plates and steel plates faced on both sides with a friction material such as compressed paper.
The functional fluid used in automatic transmissions is subjected to very severe conditions of use. The temperature of the automatic transmission fluid under normal operating conditions will reach 275.degree. F. Under more servere conditions, such as during climbing hills, trailer towing, stop-and-go traffic in the metropolitan areas, etc., the fluid temperature can increase significantly above this, up to, for example 325.degree. F and higher. In addition, the fluid is constantly being pumped and agitated, thereby being brought into intimate contact with the atmosphere within the automatic transmission. Fresh air and atmospheric moisture are constantly introduced through the transmission housing breather tube.
An additional problem has arisen in automatic transmissions in that, when a copper brazing alloy containing from about 7 to 8% phosphorus, about 5 to 7% silver, and less than 1% trace elements is used in the transmission fluid cooling system, severe corrosion can occur. This corrosion can cause leakage which damages the transmission and can necessitate its replacement.
It is an object of this invention to provide a functional fluid which prevents or retards corrosion of alloys which are contacted by said functional fluid. A further object of this invention is to provide a functional fluid which prevents or retards corrosion of a brazing alloy having the composition as defined above.
2. Description of the Prior Art
Closely related lubricating oil compositions which do not prevent or retard corrosion in the brazing alloy described above are described in U.S. Pat. No. 3,920,562.